The present invention relates generally to thermoplastic elastomer compositions. More particularly, the present invention relates to thermoplastic olefin alloys of olefin copolymer resins and olefin copolymer elastomers.
Rubber products have generally found extensive use in applications which require elasticity and flexibility. Molding of rubber into a finished product entails a curing step, generally referred to as vulcanization, which requires the use of specialized molding machines, long cycle times and a number of complicated processing steps. The rubber molding process, therefore, does not lend itself to mass production due to these processing difficulties. It would be highly desirable to find a rubber substitute which has the desirable properties of rubber without the need for a vulcanization step.
Many attempts have been made to find such rubber substitutes. For example, flexible plastics such as flexible vinyl chloride resins, ethylene/vinyl acetate copolymers and low density polyethylenes generally have good flexibility, fabrication and molding properties, but suffer from poor heat resistance, impact strength and resiliency (rebound) which greatly restrict their utility.
In order to improve the properties of such flexible plastics, they have been blended with high melting point plastics such as high density polyethylene and polypropylene. This blending, however, causes a loss in flexibility. Also molded articles of good quality cannot be produced due to flowmarks, sinkmarks and other imperfections which may occur during the molding process.
More recently, a class of compounds having properties between those of cured rubbers and soft plastics are being investigated. These compounds are generally referred to as thermoplastic elastomers (TPE). The classical TPE structure involves a matrix of an elastomer such as, for example, a polybutadiene, polyester or polyurethane, with a crosslinked network tied together by thermoplastic junction regions. A well known example of a TPE is Shell's Kraton.RTM. G, an SBS triblock of styrene and hydrogenated polybutadiene, where the thermoplastic crosslinking points are small domains of glassy polystyrene held together by polybutadiene blocks. This structure leads to behavior similar to vulcanized elastomers but, at temperatures above the polystyrene softening point, the system undergoes plastic flow.
A subset of thermoplastic elastomers, embodying only olefin based polymers, is referred to as thermoplastic olefins (TPO). A typical TPO comprises a melt blend or like mixture of a polyolefin resin, generally polypropylene, with an olefin copolymer elastomer (OCE). The polyolefin resin will give the TPO rigidity and temperature resistance while the elastomer imparts flexibility and resilience as well as improving the toughness of the material.
TPOs find particular application in the auto industry for flexible exterior body parts such as, for example, bumper covers, nerf strips, air dams and the like. In such applications, it is desired that the TPO have good resiliency (ability of the part to return to its original shape after deformation), impact strength at low temperatures, flexibility, high heat distortion temperature, surface hardness and surface finish characteristics. Additionally ease of processability and molding is desired.
Other applications for TPOs include films, footwear, sporting goods, electric parts, gaskets, water hoses and belts, to name just a few. Particularly in films, elasticity and clarity properties are important. Other of the aforementioned properties will be important depending upon the desired application.
The prior art discloses a wide variety of TPOs and processes for producing the same. For example, U.S. Pat. Nos. 3,806,558 and 4,143,099 (both incorporated by reference herein for all purposes as if fully set forth) teach a TPO comprising a blend of an olefin copolymer elastomer, typically an ethylene-propylene or ethylene-propylene-nonconjugated diene elastomer, with a polyolefin resin, typically polypropylene. The blend is produced by mixing these two components in the presence of an organic peroxide curing agent to partially cure the elastomer.
In order to yield such a TPO with good flexibility and heat distortion resistance, however, it has been necessary to use 50% or more by weight of the OCE. This high OCE content produces a TPO which is not very suitable for injection molding due to poor melt flow properties resulting in flow lines, weld lines and other surface imperfections in the molded parts. Additionally, the OCE is a more expensive component of the TPO, and the use of less OCE is highly desirable to lower product costs.